Power and wavelength polarization bistability with very wide hysteresis cycles in a 1550nm-VCSEL subject to orthogonal optical injection

We have measured optical power and wavelength polarization bistability in a 1550nm-Vertical Cavity Surface-Emitting Laser (VCSEL) subject to orthogonally-polarized optical injection into the orthogonal polarization of the fundamental transverse mode. Optical bistability with very wide hysteresis cycles, up to four times wider than previously reported results has been measured for both the optical power and wavelength domain. We also report the experimental observation of three different shapes of polarization bistability, anticlockwise, clockwise and X-Shape bistability, all of them with wide hysteresis cycles. This rich variety of behaviour at the important wavelength of 1550 nm offers promise for the use of VCSELs for all-optical signal processing and optical switching/routing applications. ©2009 Optical Society of America

Nonlinear dynamics induced by parallel and orthogonal optical injection in 1550 nm Vertical-Cavity Surface-Emitting Lasers (VCSELs)

We report a first experimental study of the nonlinear dynamics appearing in a 1550 nm single-mode VCSEL subject to parallel and to orthogonal optical injection. For the first time to our knowledge we report experimentally measured stability maps identifying the boundaries between regions of different nonlinear dynamics for both cases of polarized injection. A rich variety of nonlinear behaviours, including periodic (limit cycle, period doubling) and chaotic dynamics have been experimentally observed. ©2010 Optical Society of America

Transverse Mode Switching and Locking in Vertical-Cavity Surface-Emitting Lasers Subject to Orthogonal Optical injection

©2007 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.In this paper, we report on theoretical and experimental investigation on polarization and transverse mode behavior of vertical-cavity surface-emitting lasers (VCSELs) under orthogonal optical injection as a function of the injection strength and of the detuning between the injection frequency and the free-running frequency of the solitary laser. As the injection strength increases the VCSEL switches to the master laser polarization. We find that the injection power necessary to obtain such polarization switching is minimum at two different values of the frequency detuning: the first one corresponds to the frequency splitting between the two linearly polarized fundamental transverse modes, and the second one appears at a larger positive frequency detuning, close to the frequency difference between the first-order and the fundamental transverse modes of the solitary VCSEL. We show theoretically that both the depth and the frequency corresponding to the second minimum increase when the relative losses between the two transverse modes decrease. Bistability of the polarization switching is obtained for the whole frequency detuning range. Such a bistability is found for the fundamental mode only or for both transverse modes, depending on the value of the detuning. The theoretical and experimental optical spectra are in good agreement showing that the first-order transverse mode appears locked to the external injection

A comparative study of Sinhalese literature in the eleventh and twelfth centuries.

The thesis consists of six chapters. The first chapter deals with the historical background of the period (eleventh and twelfth centuries A.D.). It sums up the results of the Cola invasion and examines the nature and the functions of intellectual awakening after the re-establishment of Sinhalese rule with special reference to the age of Parakramabahu I. The second chapter treats briefly almost all the works of the period with their respective authors. It includes various categories of works', literary and otherwise, written in Sinhalese, Pali and Sanskrit. The third chapter concerns the concept of bhakti and its special significance in the context of devotional literature. It also shows how this concept was transferred to the Buddha by later Buddhist devotee writers. The fourth chapter makes a comparative study of the works of Gurulugomi and Vidyacakravarti, It examines their inspirations and influences from such Buddhist works as Avadanas as well as Sanskrit poetic traditions and some Sanskrit prose works. It also assesses the literary quality of each work. The fifth chapter contains a comparative study of the Sasadavata and the Muvadevdavata. It deals with the concept of poetry with special reference to the Siyabaslakara, and examines the inspiration of these Sinhalese poems from Buddhist literary traditions in addition to their influence from Sanskrit poetic traditions. It also approaches each work aesthetically following the rasa theory as postulated by Anandavardhana and Visvanatha. The sixth chapter deals with the problem of literary translation. It examines the translating processes put into practice by Ceylonese writers from early times. It pays special attention to the translating processes practised by Gurulugomi and Vidyacakravarti in their renderings of verse and prose

Stability of the nonlinear dynamics of an optically injected VCSEL

Automated protocols have been developed to characterize time series data in terms of stability. These techniques are applied to the output power time series of an optically injected vertical cavity surface emitting laser (VCSEL) subject to varying injection strength and optical frequency detuning between master and slave lasers. Dynamic maps, generated from high resolution, computer controlled experiments, identify regions of dynamic instability in the parameter space. © 2012 Optical Society of America

Physics and Applications of Laser Diode Chaos

An overview of chaos in laser diodes is provided which surveys experimental achievements in the area and explains the theory behind the phenomenon. The fundamental physics underpinning this behaviour and also the opportunities for harnessing laser diode chaos for potential applications are discussed. The availability and ease of operation of laser diodes, in a wide range of configurations, make them a convenient test-bed for exploring basic aspects of nonlinear and chaotic dynamics. It also makes them attractive for practical tasks, such as chaos-based secure communications and random number generation. Avenues for future research and development of chaotic laser diodes are also identified.Comment: Published in Nature Photonic

Genomic sequencing of SARS-CoV-2 in Rwanda reveals the importance of incoming travelers on lineage diversity

COVID-19 transmission rates are often linked to locally circulating strains of SARS-CoV-2. Here we describe 203 SARS-CoV-2 whole genome sequences analyzed from strains circulating in Rwanda from May 2020 to February 2021. In particular, we report a shift in variant distribution towards the emerging sub-lineage A.23.1 that is currently dominating. Furthermore, we report the detection of the first Rwandan cases of the B.1.1.7 and B.1.351 variants of concern among incoming travelers tested at Kigali International Airport. To assess the importance of viral introductions from neighboring countries and local transmission, we exploit available individual travel history metadata to inform spatio-temporal phylogeographic inference, enabling us to take into account infections from unsampled locations. We uncover an important role of neighboring countries in seeding introductions into Rwanda, including those from which no genomic sequences were available. Our results highlight the importance of systematic genomic surveillance and regional collaborations for a durable response towards combating COVID-19.info:eu-repo/semantics/publishe

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Polarization switching, locking and synchronization in VCSELs with optical injection

Le laser à cavité verticale émettant par la surface (VCSEL ou Vertical-Cavity Surface-Emitting Laser) comporte des avantages compétitifs par rapport aux lasers émettant par le côté. Le VCSEL présente souvent deux modes de polarisation linéaire orthogonaux avec des fréquences et des gains optiques presque identiques. Dès lors, de faibles perturbations telles que des modifications du courant d injection ou de la température peuvent induire des basculements de polarisation. Toutefois, en utilisant un schéma d injection optique d un faisceau laser externe, il est possible de contrôler ces instabilités de polarisation. Nous nous intéressons au contrôle du basculement de polarisation, à la bistabilité de polarisation ainsi qu à la compétition des modes transverses d un VCSEL soumis à injection optique de polarisation orthogonale. En variant les paramètres d injection¡(puissance injectée et désaccord en fréquence entre le laser maître et le VCSEL), nous montrons que le basculement de polarisation implique des dynamiques non linéaires telles le mélange d ondes, le cycle limite, la résonance subharmonique ainsi qu une route de doublement de période vers le chaos optique. L analyse des bifurcations sous-jacentes permet de dresser une cartographie des dynamiques de basculement de polarisation du VCSEL. orthogonaux affecte la qualité de la synchronisation du chaos. Ces résultats sont intéressants dans le cadre du développement de liaisons de communication sécurisée par chaos optique. Dans notre thèse, nous étudions également la synchronisation du chaos de VCSELs dans un schéma de couplage unidirectionnel. Nous montrons que la compétition des modes de polarisation linéaireVertical-cavity surface-emitting lasers (VCSELs) have competitive advantages with respect to conventional edge-emitting lasers such as the fabrication of dense two-dimensional VCSEL arrays for optical interconnects. VCSELs exhibit intriguing polarization properties which are critical in polarization-sensitive applications. Polarization switching (PS) between two VCSEL s preferential orthogonal linearly polarized (LP) modes may be induced by changing the bias current, temperature or, externally, through orthogonally polarized optical injection. Depending on optical injection parameters, i.e., the injected power and the frequency detuning, injection-locking of the VCSEL can be achieved. We contribute to the study of polarization bistability, polarization switching (PS), injection-locking and transverse mode competition in a VCSEL subject to orthogonal continuous-wave optical injection. We experimentally show and theoretically demonstrate that PS and locking may involve rich nonlinear dynamics including wave mixing, limit cycle, subharmonic resonances or a period-doubling route to chaos. In particular, we analyze the interplay between PS and the underlying nonlinear dynamics, and unveil its bifurcation mechanisms. Our study brings a new insight into the physics of polarization dynamics in externally-driven VCSELs. Chaos synchronization of coupled VCSELs is also investigated. A feedback-induced chaotic light from the master VCSEL is unidirectionally injected into the slave VCSEL. We show that the synchronization quality can be significantly enhanced when chaos involves both orthogonal LP modes. This result is interesting for chaos communication schemes based on VCSELs.METZ-SCD (574632105) / SudocSudocFranceF